1. Field of the Invention
The present invention relates to type I nanocomposites. More particularly, the present invention relates to type I nanocomposites useful as permeable reactive barriers to selectively remove metal ion contaminants from subsurface contaminated water.
2. State of the Art
Subsurface contaminated groundwater containing cesium or other metals presents an important environmental problem that must be addressed. Several approaches have been used to remediate contaminated groundwater. For instance, research into subsurface walls that prevent the spread of contaminants through the subsurface has been ongoing since about 1990 at Laurence Berkeley National Laboratory (LBNL). These walls, called viscous liquid barriers (VLBs), are designed to control subsurface contaminants by forming an impermeable barrier, allowing nothing, either benign or hazardous, through the wall. VLBs are composed of liquid ceramic precursors that fast-cure in the subsurface to produce a purely ceramic monolith with extremely low permeability. In laboratory tests these walls functioned well, yet during field tests problems were discovered. Specifically, diversion of groundwater flow patterns prevented this technology from being effective.
Another technology that has been used to remediate contaminated groundwater is permeable reactive barriers (PRBs). Generally described, PRBs are underground permeable walls with a reactive material (e.g., iron, compost, limestone, sodium dithionite, or zeolites) that degrades or immobilizes contaminants in groundwater flow. As contaminated water passes through the reactive zone of the PRB, the contaminants are either immobilized or chemically degraded to a more desirable state (e.g., less toxic, more readily biodegradable, etc.). PRBs are installed as permanent, semi-permanent, or replaceable units across the groundwater flow path of a contaminant plume. The barriers may contain reactants, nutrients, or oxygen depending on the contaminant. PRBs can be installed in one of two basic ways: funnel-and-gate or trench. The funnel-and-gate system has impermeable walls that direct the contaminant plume through a gate containing the reactive media. In the trench technique, a trench is installed across the path of the plume and is filled with the reactive media. In both cases the groundwater is able to pass through the media while the contaminant is collected.
The most widely used PRB in remediation projects is the zero valent iron (ZVI) wall. This type of wall, made from iron or an iron containing material, can be placed in the ground in various ways such as, conventionally or via slurry injection, depending upon the demands of a particular site. The ZVI wall functions to remove chlorinated organic contaminants from the subsurface by reductively de-chlorinating these species as contaminated groundwater flows through the wall. While this is an extremely effective remediation technology for halogenated organic contaminants, ZVI walls are not able to effect the removal of most metal ions from subsurface contaminant plumes. Despite these limitations, PRBs are still a relatively new remediation strategy, and offer tremendous advantages once the current technical difficulties are overcome. Examples of permeable barriers for decontaminating groundwater using iron-based or other materials such as active metals, activated carbon, limestone, etc are disclosed in U.S. Pat. No. 6,254,786 to Carpenter et al. and U.S. Pat. No. 6,428,695 to Naftz et al.
While the ZVI wall and VLBs are capable of remediating groundwater, a need exists for a PRB capable of selectively removing metal ions such as cesium from contaminated groundwater. An ideal PRB would: (1) have a tunable water passing rate to approximate the hydraulic conductivity of the subsurface environment where the PRB is placed; (2) have sufficient mechanical strength, when wet and dry, to maintain barrier integrity; (3) have the ability to incorporate selective metal sequestration agents so that they remain active, yet do not leach from the barrier; and (4) be deployable through direct injection methods such that trenching is not needed. Additionally, there is a need to keep the technology as low cost as possible, while remaining reliable. The present invention, as described in more detail below, fulfills these needs.
The inventors fulfill the above mentioned needs by creating new nanocomposite materials that may be used to form PRBs of the present invention. Generally, type I nanocomposites have a preformed polymer constituent embedded in an inorganic constituent wherein the inorganic constituent is formed in situ from the condensation of an inorganic precursor in a mixture with the polymer constituent. Type I nanocomposites also lack significant covalent bonding between the preformed polymer constituent and inorganic constituent. Type I nanocomposites have been reported to be formed with a variety of organic preformed polymers and inorganic precursors. Namely, nanocomposites formed from preformed polymers such as polyacrylonitrile (PAN), polyethyleneoxide (PEO), polyethylene glycol (PEG), polyvinyl acetate (PVAc), polyvinyl alcohol (PVA) and tetraethylorthosilicate (TEOS) are known in the art to be formed from the condensation reaction of the TEOS in the presence of the solvated polymer. Such material systems form nanocomposites having a polymer constituent (PAN, PEO, PEG, PVA, or PVAc) forming an interpenetrating network with the silicon dioxide constituent. Other types of preformed polymers have been used with TEOS to form nanocomposites. The article Organic/Inorganic Hybrid Network Materials by the Sol-Gel Approach, Chem. Mater. 1996, 8, 1667–1681 by J. Wen and B. L. Wilkes provides a background on many types of nanocomposites that have been formulated.